On a pleasure boat or a competition vessel the mainsail is guided in a rail disposed along the mast, where it is kept under tension when it is in the hoist position. This tension is applied to the sail in the first place by the halyard which permits the hoisting of it and in the second place, at the lower end of the sail, by a downhaul, generally composed of a pulley block with a plurality of sheaves. The halyard, which is attached by means of a spring hook or a shackle to an eye which is provided in the head of the sail, passes over a pulley pivotably mounted on the upper part of the mast and returns along the mast, on the interior or exterior of it as far as a winding and securing device.
During navigation the wind, which is the element propelling the boat, exerts a pressure on the entire sail. This pressure is represented by forces applied to the mast which may reach several hundreds of daN. These forces are applied to both the halyard and the downhaul. To prevent the halyard from becoming longitudinally stretched under the action of this pressure and reducing the tension applied to the sail halyards composed of stainless steel cables or of a synthetic material with a low coefficient of elongation, such as a material known under the name of KEVLAR, are generally used. Now, stainless steel halyards are difficult to use and Kevlar halyards are very costly and require careful handling.
It was therefore envisaged fixing at the mast head, as shown in FIGS. 1 and 2, a mounting 2 with a wing projecting from the mast 3. Between the head of the sail 4 and the halyard 5 actuating this there is interposed, in addition to a shackle 6 a piece 7 of the general shape of the letter S.
When the sail is fully hoist by means of the halyard 5, which passes over a pulley 8 disposed at the mast head, the piece 7 hooks onto the mounting 2 and assures that the sail is held in this position. The halyard, which then no longer has to exert a tensile force on the sail, may be made of a textile material or of an extensible synthetic material such as nylon.
The disadvantages of this system, which is shown in frontal and side view in FIGS. 1 and 2, is that it is difficult to detach the piece 7 from the mounting 2 to haul down the sail. It is necessary, in fact, first to pull on the halyard 5 to release the attaching means 7 from the mounting 2, and then to pull laterally on the sail, or, if possible, to rotate the mast to bring the piece 7 beside the mounting 2. This is a delicate operation which limits considerably the use of this type of device.